Up to now, API tool joints have been widely employed as threaded joints for drill pipes used in drilling oil and gas wells of up to several kilometers in depth. As FIG. 2 shows, the general-purpose API tool joint (hereinafter called general-purpose tool joint) is provided with a female threaded portion 14 at one end of tubular body 12. Conversely, tubular body 11 is provided with a male threaded portion 13 and an external shoulder 15 at one end. These tubular bodies are designed so that external shoulder 15 contacts the tip 16 of the tubular body 12 when the male threaded portion 13 and the female threaded portion 12 are connected.
The said general-purpose tool joint is designed to transmit torque at the time of drilling through the external shoulder and has been widely used for connecting drill pipes used in drilling oil wells.
Tool joints featuring two shoulders have been previously proposed, for example, in JP,A No. 58-152992, JP,B Nos. 58-25632, 59-11063, JP,A No. 63-501167, but the number of these tool joints used has been quite limited as compared with the above general-purpose tool joint.
In the case of the aforementioned tool joints, the external shoulder and friction torque at the thread connection portion bear the torque. Thus, if excessive torque occurs at the bottom of a well during drilling, the external shoulder will yield, causing the tubular body 12 to expand in bell-shaped deformation, or resulting in breakage of the thread portion as pointed out in the Drilling Manual and other documents.
The API standard states that the strength of the abovementioned tool joint should be more than 80% of the torsional strength of the pipe. In the case of high-grade pipes, however, the strength does not satisfy the standard resulting in the occurrence of various troubles.
Also, tool joints other than the abovementioned general-purpose tool joints are not interchangeable with general-purpose types and cannot been used with general-purpose drill pipes already prepared at drilling sites for drilling wells up to several kilometers deep. Thus, they are not adopted for practical drilling applications.
The tool joints disclosed in the above JP,A No. 58-152992, etc. have peculiar and complicated threads. As a result, the use of such tool joints requires extra work processes, leading to longer construction periods.